(a) Field of the Invention
The present invention relates generally to substantially halogen-free (e.g., less than or equal to 500 weight parts halogen per million weight parts polyimide) polyimide based compositions useful as sheet adhesives, and/or coverlay compositions, in flexible printed-circuit board applications.
More specifically, the substantially halogen-free compositions of the present invention comprise at least: i. a low modulus polyimide derived from a dianhydride component and a diamine component where from 15 to 60 mole percent of the diamine component is a diamine having a polysiloxane moiety; ii. an epoxy (or a functional derivative of an epoxy) adjuvant; iii. a plasticizer; and iv. a flame-retardant filler.
In one embodiment of the present invention, the substantially halogen-free composition optionally comprises an adhesion promoter.
The compositions of the present invention can generally be applied to other substrates or materials at low lamination temperatures. These compositions can typically provide excellent heat resistance (including flame retardancy), flexibility, dimensional stability and resistance to curl.
(b) Description of the Related Art
Broadly speaking, coverlay films are used to protect circuits supported on flexible printed-circuit boards. Conventional coverlay films are often prepared by coating one surface of a base film, such as a polyimide film, with an adhesive, such as an acrylic adhesive or an epoxy adhesive.
Conventional coverlay films can be problematic, because the epoxy (or acrylic portion) of the coverlay composition will oftentimes diminish the heat resistance and/or other useful properties of the film. In addition, epoxies containing bromine are commonly used for good flame-retardance, however current industrial applications are now favoring the use of substantially halogen-free flexible circuit materials.